Lipids of sulphur-oxidizing bacteria isolated from Thyasim flexuosa had large amounts of 18:l(n-7) together with lesser amounts of both 16:l(n-7) and 16:0 as their major fatty acids. Smaller amounts of the cyclopropyl fatty acids, cyclic δ9,10 C16 and especially cyclic δll,12 C18 were also present. A similar pattern was found for methylotrophic bacteria isolated from Thyasira, Myrtea and Lucinoma, except that 16:l(n-7) and 16:0 were both present in larger amounts than 18:l(n-7) and small amounts of cyclic δ9,10 C16 and lesser amounts of cyclic All,12 C18 were present in all cases. The fatty acids 18:l(n-7), 16:l(n-7), 16:0, cyclic δ9,10 C16, and cyclic δll,12 C18 were all present in varying amounts in several free-living, non-marine, sulphur-oxidizing bacteria analysed, and in one bacterium both cyclic δ9,10 C18 and cyclic δll,12 C20 were abundant. Branched-chain fatty acids and polyunsaturated fatty acids were not detected in any of the sulphur-oxidizing or methylotrophic bacteria analysed. The lipids of the sulphur-oxidizing and methylotrophic bacteria consisted largely of phosphatidylethanolamine, phosphatidylglycerol and cardiolipin. The results are discussed in terms of fatty acids as indicators of nutritional relationships in bacterial-invertebrate symbioses.